Chemical and pharmaceutical industries are constantly seeking new ways towards sustainable chemistry that allows for less waste production and reduced energy consumption during industrial processes. One way, which I investigated in my PhD research, would be to use nanoparticles to speed up chemical reactions – a process called catalysis.
Nanoparticles are tiny objects or combinations of several atoms that can be as small as 1 nm and show unique properties, different from the bulk material. Take gold, for example. We are used to its shapes and colors in jewelry, but gold nanoparticles show completely new properties, such as red color in solution or the ability to behave as a catalyst in an efficient and selective manner. Similarly, nanoparticles of Platinum (Pt) and Palladium (Pd) can also act as catalysts in various processes (i.e. hydrogenation, the reduction of organic compounds). Nanoparticles promise to be highly selective, to greatly increase the reaction rate and to lower energy consumption. However, one major disadvantage of nanoparticles is the tendency for aggregation and thus the loss of their unique catalytic properties.
MOFs support nanoparticles for catalysis
In my PhD, I successfully encapsulated catalytically active metal nanoparticles of Pt or a combination of Pt/Pd into stabilising support materials called metal-organic frameworks (MOFs), which prevent aggregation. Inside MOFs, the tiny metal particles could maintain their unique capacity to hydrogenate nitrobenzene-based compounds and additionally be very selective towards the target products. Moreover, I could show that the combination of bimetallic nanoparticles of Pt and Pd with MOFs can surpass the catalytic hydrogenation activity of the monometallic Pt nanoparticles. This alternative solution may help to reduce the costs for the use of noble metals, replacing parts of the expensive Pt by Pd.
Especially in view to catalysis, MOFs exhibit two beneficial properties: Storage capacity (sponge-like property) and molecular selectivity (sieve-like) – in fact, due to the unique microporous structure of the support, the embedded nanoparticles are only accessible by molecules that fit the dimensions of the MOF pores (see Figure 1). MOFs are porous materials, formed by interconnection of organic linker molecules and metal ions or clusters. Thereby, a three dimensional network with a huge inner surface area is formed, which simultaneously possesses enough space to accommodate small sized nanoparticles or other guest molecules and solvents (water, nitrogen, carbon dioxide). Hence, MOFs feature the ability to absorb and/or separate a certain amount of substances at the molecular level, very similar to sponges or sieves.
In my work I choose the Zirconium-based metal organic framework named UiO-66, which appears in a 3-dimensional structures with tetrahedral and octahedral pore geometries. A major advantage of this particular MOF is its extraordinary high thermal and chemical stability against water, acids and several organic solvents. Therefore, UiO-66 represents an appropriate candidate for heterogeneous catalysis, while other MOFs would decompose during the applied catalytic conditions.
Low-cost bimetallic Pd/Pt into MOFs show promising catalytic activity
Following a template approach, we exclusively embedded preformed mono- and bimetallic Pt and Pd/Pt nanoparticles without undesired deposition at the outer surface of the porous material, which in fact represents the key feature for shape-selective catalysis. Then we elucidated the structural integrity of the material and the exact spatial distribution of the nanoparticles (fully embedded into the MOF crystals or not). For instance, powder x-ray diffraction (PXRD) measurements indicated the crystallinity of UiO-66, even after the encapsulation process; transmission electron microscopic (TEM) measurements showed the successful and exclusive encapsulation of the nanoparticles into the core of the MOF crystals.
Afterwards the materials were further studied for selective hydrogenation of nitrobenzene-based compounds to the respective anilines. Direct comparison of the embedded Pt and Pd/Pt NPs showed a much higher catalytic activity for the bimetallic species, while the shape-selective character, originating from the microporous MOF, was maintained. Hopefully, this may become another possible solution towards sustainable.
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